1. The Field of Invention
The present invention concerns a method and apparatus for equally dividing vapor and liquid phases of wet steam at T junctions.
2. The Prior Art
In most steam flow situations, in which wet steam from a source or generator is distributed through a pipe network to various users, there is a phenomenon known as "Phase Splitting". This means that the liquid and vapor phases of the steam separate with the lighter vapor generally moving axially within the pipe at high velocity and the heavier liquid becoming annular and forming a liquid film which travels along the inside wall of the pipe. Thus the liquid is substantially evenly distributed around the internal circumference of the pipe. When such phase-separated steam encounters a pipe junction, such as a T junction used to distribute the steam to different locations for use, the result may be steam of differing quality exiting the respective arms of the junction. In a large steam system the wet steam flow may encounter many T junctions before finally being utilized. Phase spitting compounds itself and, in the worst case, results in some portions of a system receiving as much as 100 percent vapor whereas other portions of the system receive as much as 100 percent liquid.
Since most existing distribution systems suffer from the problem of phase splitting, a simple and inexpensive device to eliminate phase splitting, without having to redesign the entire steam distribution system, is greatly needed. One commercially available device, see U.S. Pat. No. 4,824,614, the disclosure of which is incorporated herein by reference, claims to substantially eliminate problems associated with phase splitting in a branch T junction. A branch T junction is one in which a portion of the incoming flow is taken off through an outlet having an axis extending normal to the axis of the inlet, and the remaining flow continues straight through the junction to a second outlet, as opposed to an impacting T junction where the incoming flow is split between two coaxial and oppositely directed outlet branches. This prior art device only succeeds in eliminating phase splitting in a branching T junction when the inlet mass is equally split between downstream legs of the T junction; that is 50 percent of the inlet mass is extracted in each outlet leg of the T junction. All other mass extraction ratios result in phase splitting. Most T junctions in a steam system operate at conditions other than a mass extraction ratio of 50 percent, therefore, phase splitting would still be present at most junctions.
This prior art device includes a horizontal "static mixer" spool installed at the inlet of the T. Theoretically, the fluid exits the mixer spool as a "homogeneous" mixture. The mixture then enters a "flow stratifier" section which includes several vertical layers of alternate flow division chambers to reduce the establishment of secondary flow currents. Finally, the mixture encounters a "dividing wall" which guides the flow to its appropriate downstream outlet.
The reason the device only works at an extraction ratio of 50 percent is thought to be a result of equal pipe cross sectional areas (50 percent each) allocated at the dividing wall. If the flow is truly homogeneous after the static mixer, then the liquid phase is evenly distributed throughout the vapor phase. If for example, 20 percent of the vapor is taken off into the branch leg of the T junction, 20 percent of the liquid phase must be also taken off in order to result in a branch steam quality (and run steam quality) equal to the inlet steam quality. In order for this to occur, 20 percent of the pipe cross sectional area must be allocated to the branch side of the dividing wall. The reason for this is because as the two-phase mixture passes through the inlet static mixer, the liquid phase is accelerated into the vapor phase and is evenly distributed throughout the vapor phase. With the acceleration of the liquid phase comes an increase in the momentum of the liquid phase. It is extremely difficult for the high momentum liquid to be taken off 90 degrees into the branch leg of the T junction. The concept of the dividing wall allows one to take off liquid into the branch, however, since the flow cross sectional area is fixed at 50 percent, the fraction of liquid that will be taken off into the branch remains relatively constant at approximately 50 percent. Slight variations may be attributed to the interfacial drag forces imposed on the liquid phase by the vapor phase.
Another patented device, see U.S. Pat. No. 4,269,211, the disclosure of which is incorporated herein by reference, claims to substantially eliminate problems associated with phase splitting in a T branch junction. This device uses a retractable, circular, perforated disc to divert the flow of liquid into the branch of the T. The disc is inserted into the center of the T along the axis of the branch. The device is unsuccessful in diverting the liquid phase of the steam into the branch because the disc is in contact with the wall of the T junction for only half of the pipe's circumference. Because the liquid phase of the steam is generally distributed as a circumferential film in contact with the wall of the pipe, the disc can influence only half of the liquid in the steam stream. Further, the perforations in the disc allow that portion of the liquid which may be diverted to the branch outlet to be blown through the perforations toward the run outlet thus further reducing the performance of the device.